The present invention relates to operators for doors, such as overhead garage doors. More particularly, the present invention relates to motor-driven door operators in which a motor, such as an alternating current induction motor, actuates a drive mechanism connected to a door to move the door between an open position and a closed position in response to activation of a control circuit. Specifically, the present invention is directed to an improved control circuit for the motor.
Typically, commercial door operators are packed and shipped as a unit. That is, the shipping carton accommodates both the motor and the drive mechanism. While installation of the door operator is facilitated, there is an economic disadvantage. In the case of a door operator of the screw type, for example, in which the drive mechanism comprises a track, a drive screw and a shuttle, the carton is very expensive due to the fact that the motor is substantially wider than the drive mechanism, but both are shipped in a carton wide enough to accommodate the motor.
In prior art door operators, after installation and connection of power or after a power outage and subsequent restoration of power, there is a proneness for loss of sequence of operation of the motor. That is, if the door is in the open position, nevertheless, upon activation of the control circuit the door is driven in the opening direction, or, if the door is in the closed position, the door is driven in the closing direction. This can result in damage to the door operator.
Additionally, for safety, many prior art door operators stop or reverse movement of the door if the closing door should engage an obstacle, such as a person or some object. Also, in order to protect the motor, some door operators are sensitive to obstacles engaged during door opening so that the motor is stoped before damage results from overheating.
The prior art door operators employ different approaches to provide increased sensitivity. Some approaches for increased sensitivity to obstructive forces, however, have met with the problem that the inertia of the door, which has a considerable weight, and of the drive mechanism at rest is such that the sensitivity must be set so that the obstruction detection means does not stop or reverse the motor each time movement of the door is initiated. Consequently, many prior art door operators are sensitive only to obstructive forces in excess of the inertia of the door and of the drive mechanism at rest. As a result, injury or damage, such as denting the hood of an automobile in the path of the door, can result before the obstructive forces exceed the inertial force to cause stopping or reversing of the motor.
One approach for increasing the sensitivity of the door operator to obstructions is shown in U.S. Pat. No. 2,533,116 which includes a dashpot to prevent activation of safety switches due to the inertia of the door and of the drive mechanism when movement of the door is initiated. The apparatus in U.S. Pat. No. 2,533,116, however, is mechanically complex and, consequently, expensive.
As an alternative, U.S. Pat. No. 4,010,408 incorporates cutoff switches to override the function of safety switches when the door is within a predetermined distance of either the open or the closed position. The inertia of the door and of the drive mechanism is overcome as the door travels the predetermined distance so that, although the door operator is not sensitive to obstructive forces within the predetermined distance, there is sensitivity to obstructive forces at other positions. Unfortunately, the door cannot be stopped and restarted at those other positions since the cutoff switches are not operative to override the safety switches in the intermediate region.
Furthermore, in typical installations, the door operator is intended to close the door to form a sealed barrier across the passageway. In many prior art door operators, however, the fully closed position is set by means of a limit switch which is activated to stop the motor. Climatic conditions, such as snow on the door threshold and temperature extremes, can result, however, in the door either being obstructed from reaching the limit switch, in which case the door is reversed, or being stopped due to activation of the limit switch before the edge of the door is sealed against the threshold.
In order to override reversal of the door before the limit switch is activated, the door operators in U.S. Pat. Nos. 2,558,808, 3,078,407 and 3,474,317 include a reverse override switch so that the door stops rather than reverses if an obstructive force is sensed within a predetermined distance of the closed position. Nevertheless, where temperature changes cause expansion or contraction of the door support structure, such as the door rails, a positive seal is not necessarily effected between the door and the threshold. Moreover, two switches, that is, a limit switch and a reverse override switch, are required which complicates installation and adds expense.
Additionally, in typical door operators, as the door moves to the open position, the motor is stopped by activation of a limit switch. Similarly, as the door moves to the closed position, the motor is stopped by activation of a limit switch. Also, as previously discussed, the motor is stopped or reversed by a safety switch if an obstacle is encountered. If the obstruction causes the door to reverse, the door moves to the open position or to the closed position, as the case may be, and the motor is stopped by activation of a limit switch.
In prior art door operators, however, if one of the switches fails, energization of the motor continues, and personal injury or property damage could result. Also, if the motor stalls due to encountering an obstruction or reaching a limit position but continues to be energized due to failure of one of the switches, damage can result from overheating.